1. Field of the Invention
The present invention relates to a method of printing raster images while economizing printing material, starting from input image data corresponding to a predetermined original format and comprising the step of forming a raster of image points in which the rows and columns are reduced by a factor n with regard to the raster in the standard mode and in which each image point defines either a point to be printed or a blank point of the rasterised image, for use with color or monochrome printers that operate by means of a raster, such as ink jet printers or laser printers, and also thermal or electrostatic printers. The invention also relates to a printer of the above-specified type enabling such a method to be implemented.
2. Discussion of Background Art
With raster-printing printers, a matrix of elementary points is created in memory on the basis of input image data. Each matrix point is indexed to a determined point of the print output raster. It has a binary value that determines whether the corresponding point in the raster is to be linked (a printed point or xe2x80x9cdotxe2x80x9d) or left white (a blank point).
Input image data is generally of three types: vectors, gray-level rasters, and binary rasters.
Data in vector form defines lines to be printed by specifying a start point and an end point in a coordinate plane.
Data in gray-level raster form defines a mosaic of image points in a matrix that is configured in rows and columns, where each element has an intensity value allocated thereto in the range of white to black.
Data in binary form is made up of rasters as in the preceding case, but there are only two possible values for each point of the image, corresponding either to a point that is printed as a dot or to a point in the raster image that is left blank. A half-tone appearance can be rendered by modulating the density of printed points.
In certain applications, it is often necessary to print several drafts for visual inspection before printing a final version. For printing drafts, it is common-place to use draft-mode printing, i.e. printing a relatively low print density so as to reduce the running cost of the printer and also extend the number of sheets it can print.
Numerous known printing methods exist for reducing consumption of printing material in an economic print mode.
For example, it is proposed in document JP-A-52 61973 to define a print zone in the form of a matrix of image points over which a patterned mask is superposed electronically to prevent certain points being printed, depending on the pattern.
In another method described in document JP-A-62 212 164, all even-numbered points in consecutive runs of image points to be printed are replaced by blank points in the row direction.
In next another method described in document EP-A-625 765, a mask pattern is generated which is applied to the image data prior to printing to reduce the number of dots actually printed.
In next another method described in document EP-A-582 434, the raster data are depleted in horizontal direction (dot reduction) and the head scans the row at higher rate, so the printed size of a page is not changed.
In methods of those types, based on eliminating in a systematic manner some of the points that are to be printed there is the risk of completely deleting from the printed output certain patterns defined by fine rows or by isolated points.
Other known methods operate by selectively eliminating points for printing by taking account of the immediate graphics environment, in order to reduce the risk of unwanted deletion.
For example, document EP-A-689 159 proposes to form a raster of image points in which the rows and columns are reduced by a factor n with regard to the raster in the standard mode each image point defining either a point to be printed or a blank point of the rasterized image, which raster is logically combined with a set of three-pixel bits held both in vertical and horizontal direction with the pixel of interest being located in the center, to perform dot reduction on the basis of detected edge patterns.
Document U.S. Pat. No. 5,390,290 proposes an ink-saving print method in which one point out of every three points to be printed is eliminated from a matrix of points according to the following rule: for three, not necessarily consecutive, points to be printed, the first point is retained, and the second and third points are eliminated and replaced by a new point situated halfway between the two points that are eliminated.
In another method described in document U.S. Pat. No. 5,270,728, seeking in particular to avoid unnecessary overlap between printed dots, every other point is eliminated in each consecutive run of points to be printed along each row, with odd rows being scanned in one direction and even rows in the opposite direction.
There also exist printing methods which seek more specifically to reduce printing time, either by spacing out dots or by eliminating a fraction of them using criteria analogous to those described above.
All of those known methods eliminate image points from the matrix of binary points formed from input graphics data. However, the process whereby a matrix of binary points is formed, particularly from graphics data in the form of vectors or rasters of gray levels, itself gives rise to a certain amount of degradation of image information, since it proceeds by approximation. Eliminating points from the matrix can only accentuate that degradation of information.
As a result, during printing, prior art techniques lead all too often to fine rows or to sets of isolated points being lost. However, it is important to be able to reproduce patterns of that kind reliably, even in print economy mode. This applies particularly to images having a high concentration of details carrying technical information, in particular in engineering, e.g. for printing electronic circuit layouts.
Furthermore, determining which points to eliminate from the matrix of points requires considerable amounts of computation time and of memory space, which adversely affects printing time.
The object of the invention is to make it possible, in a draft print mode, to print images with considerable savings of printing material, while ensuring that all of the essential elements in the supplied image data are reproduced, the method also being fast, and sparing in memory space requirements.
To this end, the method according to the preamble is characterized in that it furthermore comprises the following successive steps:
enlarging said raster of points to return to the original format by inserting blank image points in the reduced raster by a predetermined filling procedure; and
printing the image on the basis of the point raster enlarged to the original format.
It will be observed that the method of the invention makes it possible to operate on an image of reduced size, thereby reducing both the processing time necessary and memory capacity required for the step of forming the point matrix.
The savings in printing material come from the reduction of the input image which, since it has a format that is smaller in terms of image area, makes it possible to obtain a corresponding reduction in consumption of printing material. Energy savings are particularly significant with thermal printers.
Reducing the input image does indeed lead to an inevitable loss of definition. However, the image retains the essence of its information content, thereby making it possible to maintain sufficient image quality in the output print, e.g. to verify its graphics content and layout.
Also, the method does not require points for printing to be eliminated from the point matrix, as is the case with conventional printing techniques for economizing print. As a result, the graphics information is better restored in the printed image.
The input image is advantageously reduced by a scale factor of n, where n is a real number.
For a relatively simple implementation of the invention, the scale factor n can be an integer equal to or greater than 2, and preferably equal to 2.
When the scale factor n is an integer, the step of enlarging the raster may consist an integer equal to or greater than 2, and preferably equal to 2.
When the scale factor n is an integer, the step of enlarging the raster may include adding n-1 blank image point(s) between two points in each row of image points in the reduced raster, and n-1 row(s) of blank points between two rows of image points in the reduced raster.
When the input data is in the form of vectors specified in a coordinate plane, the reduction step may include dividing the definition of the input image by n.
When the input data is in the form of a raster, the reduction step may include forming a reduced raster in which each point is given a representative value based on the data in a neighborhood zone of the input raster and having topographical correspondence with the point. Several techniques that are known per se may be employed for this purpose.
In a preferred embodiment of the invention, this neighborhood zone is constituted by an nxc3x97n block of points constituted by n contiguous image points in the row direction and n contiguous points in the column direction of the input raster.
When the input raster is a gray-level raster, the representative value may be an average of the gray levels in the neighborhood zone.
When the input raster is a binary raster in which each point has one or the other of two values defining a point to be printed and a blank point, the representative value is advantageously derived from the points in the neighborhood zone in which such a manner as to avoid eliminating fine patterns. When the neighborhood zone is a 2xc3x972 point block, the representative value corresponds, for example, to a point to be printed if half or more of the points in the block are points to be printed, and otherwise it corresponds to a blank point. It is preferable, however, for the representative value to be calculated so as to give the representative value the value of a point to be printed when:
half or more of the points of the block define a point to be printed; or
one point only of the block is a point to be printed and the block is the N.m-th block in a run of not-necessarily consecutive blocks each containing only one point to be printed, where the blocks are counted by any systematic counting techniques, where N is a run of integers 1, 2, 3, . . . , and where m is a predetermined integer such that the reduced raster includes one point to be printed for each set of m input raster blocks containing only one point to be printed in each block, in order to avoid systematically eliminating isolated points to be printed in the input raster.
Under such circumstances, the number m is preferably equal to 4.
This variant is particularly effective when the binary input rasters contain numerous isolated points for printing which would otherwise run the risk of being systematically eliminated. It often happens that input binary image rasters are themselves generated by image processing in which a half-tone appearance is generated by space modulation of the points to be printed (dithering), which gives rise to rather a larger number of isolated points for printing.
Using the values n=2 and m=4, one in four of the isolated points in the binary raster will still be printed. It is thus possible to maintain a printing material saving of 75% while retaining a sufficiently large number of isolated points for them to reproduce satisfactorily the half-tone appearance of the original image.
For color printing, the steps of reducing the image, of forming the raster of points, and of enlarging the raster can be performed for each color component, and possibly also the black component, of the image to be printed.
The invention also provides apparatus for printing raster images while economizing print, based on the above method.